Method of determining wear

ABSTRACT

The method uses color to determine the wear of the screw and barrel of the molding machine. The method measures the color and/or color variation to determine the wear. A first material of a first color and a second material of a second color are fed into the machine to create a third material with a third color. The third material is either extruded to form a part or is injected into a mold to form a part. The color of the part is then measured with a measuring device, such as, spectrophotometer. The color will change as the screw and barrel wear. The color value would then be compared with previous color values to determine the amount of wear.

FIELD OF THE INVENTION

The invention relates to a method for determining wear and moreparticularly a method of using color as an indicator of the degree ofboth barrel and screw wear in plastic processing equipment such asextruders and reciprocating injection molding machines.

BACKGROUND OF THE INVENTION

Extruders and injection molding machines use a screw to feed materialthrough the machines. The screw has a flight and a channel. In addition,the screw is located inside a barrel. As the screw rotates inside thebarrel, the flight advances the material through the barrel. For plasticmaterial, the screw flight is designed to cause the material to melt asit advances through a heated barrel. In addition, the screw may be usedto mix two or more materials as they are moved through the barrel.Furthermore, the machine may use several screws.

The machine is designed and manufactured with a predetermined distancebetween the screw flight and the barrel. During use, the movement ofmaterial through the screw and barrel causes wear which increases thedistance between the screw flight and the barrel. If the material isabrasive, the wear occurs more quickly.

Generally an increase in wear creates a reduction of the shear rate onthe polymer, resulting in less mixing to occur in the barrel. Thisresults in molten polymer with poor homogeneity. A non-homogeneousproduct causes the risk of fabricating plastic parts which will fail tomeet performance criteria established on the physical properties.Preventive maintenance procedures to monitor screw and barrel wearpresently used by the industry require machines to be shut down,disassembled, and physically measured to determine actual wear. Thisprocess is time consuming, and generally requires one-half to one fullday of lost productivity and the use of high cost maintenance labor,causing the process to be expensive.

A method is needed which can be more efficient and more economical forthe determination of screw wear.

SUMMARY OF THE INVENTION

In general, the present invention provides a facile method forevaluating the extent of wear, and for determining when to repair,plastic processing equipment including a screw and barrel, by firstestablishing a color standard and then evaluating what color resultswhen color samples (made from two differently colored thermal plastics)are processed in such equipment at spaced intervals over time. Becauseof the decreasing homogeneity that results as the mixing in suchequipment deteriorates over time through wear, the resulting color willcorrespondingly deteriorate in comparison to the color standard and inrelation to color samples obtained using equipment at earlier times. Thecolor of the samples can be subjectively and/or objectively evaluated toprovide a basis for determining when the equipment should be repaired.

Pursuant to one illustrative embodiment, a first material of a firstcolor and a second material of a second color are fed into the machineto create a resultant third material with a third color. The resultantthird material is either extruded or is injected into a mold to form apart.

More specifically, in this embodiment, a precolored thermoplasticcompound is fed into a machine with a custom formulated concentrate (ormaster batch) of a different color. When these two products are mixedhomogeneously, these products will produce a fabricated plastic partuniformly colored in an entirely different shade than the colors ofeither component.

The color of the part is then measured with a measuring device, such as,for example, a spectrophotometer. The color will change as the screw andbarrel wear. The color value obtained may then be compared with previouscolor values, obtained at other times during the equipment use, todetermine the amount of wear.

Specifically, the method would be performed when the machine is new (orfirst used or repaired) in order to obtain an initial or first colorvalue. The first value would be placed on a graph with operating timealong the X axis and color along the Y axis. After the specifiedinterval of operating time, the method would be performed again toobtain a second color value and this second value would be placed on thegraph. Similarly, the method would be performed after each specifiedinterval of operating time to obtain the third value, fourth value, etc.The values would then be connected with a line to form a curve. The userwould analyze the curve to determine when the color and wear hasdiminished below an acceptable level. The machine would then bedisassembled and rebuilt.

In this fashion, a straightforward method for evaluating plasticprocessing equipment wear is provided which avoids the expense and downtime of prior techniques. This method can be carried out in a number ofdifferent ways, providing as rigorous and objective analysis as the userconsiders appropriate for the particular equipment repair sequence.Moreover, this method can take advantage of color compounders who havethe ability to insure that the proper color samples are used each timefor this comparative method, as well as to provide the color changeanalysis and determination, as the equipment user deems appropriate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial schematic cross-sectional view of an extruder or aninjection molding machine.

FIG. 2 is a graph showing the wear of the machine by plotting color (C*(chroma)) versus specified intervals of operating time.

FIG. 3 is a graph showing the wear of the machine by plotting color (b*)versus specified intervals of operating time.

FIG. 4 is a perspective view of a part showing some of the locationswhere color may be measured to obtain differences in color.

FIG. 5 is a graph showing the wear of the machine by plotting color (C*(chroma)) versus specified intervals of operating time for processingdifferent types of material.

DESCRIPTION OF THE EMBODIMENTS

The invention involves a method which uses color to determine the wearof the screw and barrel. The method can be performed quickly and doesnot require disassembling the machine or the measurement of itscomponents. The method measures the color and/or color variation todetermine the wear.

In a precolored thermoplastic compound, the colorant has beenhomogeneously distributed throughout the polymer during the meltcompounding process known as precolor compounding. This compound can bechanged to a new color by adding a new colorant. For example, a precoloryellow compound may be changed to a green compound simply through theaddition of a blue colorant to the formulation. Therefore, a standardfor optimum homogeneity can be established by preparing a precoloredgreen compound, based on both the yellow and blue colorants via themelt-compounding process.

This invention utilizes the principle that normal and continuous use ofplastic processing equipment erodes the mixing capability inherent attime of equipment purchase. Thus, as the wear continues, the capabilityof the equipment to deliver melt homogeneity is also compromised.

A homogenous precolor plastic compound is mixed with a concentrate (ormaster batch) of a second color. A resultant third material with aresultant third color is created and the color of this third color willdepend on its resulting homogeneity.

The color concentrate includes a colorant. The colorant may be apigment, a dye, a combination of pigment and dye, or other coloredmaterial.

A precolored yellow thermoplastic compound provides a “background color”that is homogeneous. The addition of an “accent color” such as blue inthe form of a concentrate (or master batch) requires complete mixing toattain the homogeneity of the resultant third material which is green.Less than complete mixing will leave “islands” or “swirls” of theconcentrate in the resultant third material. In other words, the resultis a non-homogeneous background color. If all of the colorant containedin the concentrate was uniformly mixed into the homogeneous precolorcompound (background color), the resultant third color will equal thecolor of the standard developed as the homogeneous standard.

If the mixing is compromised and the concentrate (the accent color) isnot spread homogeneously throughout the homogeneous precolored compound,the third color will be less than the color of the homogeneous standard.The further the mixing action is compromised, the greater the loss ofthe third color. The amount of colorant or the like in the concentrateremains identical for each test. Thus, the loss of mixing capability inthe processing equipment produces molten polymer containing “islands” or“swirls” of incompletely mixed colorant. This non-homogeneity of thethird color manifests itself through a change in color.

FIG. 1 shows certain portions of an extruder or a reciprocatinginjection screw machine 50. The machine 50 includes a first hopper orfeeder 52, a second hopper or feeder 54, a barrel 56 and a screw 58. Thescrew 58 has a flight 60 and a channel 62. The screw 58 is locatedinside the barrel 56 and the screw flight 60 is separated from thebarrel 56 by a distance 70. The machine is designed and manufacturedwith a predetermined distance 70 between the screw flight 60 and thebarrel 56. The screw 58 rotates inside the barrel 56 and the screwflight 60 advances the material through the barrel 56. The material isthen fed through a die 76 or injected into a mold for injection molding.

The method for determining wear is performed as follows. A firstmaterial 100 of a first color and a second material 102 of a secondcolor are fed into the machine to create a third material 104 with athird color. For example, the first material 100 may be plastic pelletswhich are yellow. The second material 102 may be a color concentratewhich is blue. The color concentrate may be in a pellet form or may bein a liquid form. The yellow material mixes with the blue material tocreate a resultant third material 104 which is green.

Specifically, the first material is a precolored thermoplastic polymerwhich is fed into a machine with a second material which is a customformulated concentrate (master batch) of a different color. Theconcentrate may be in the solid pelletized state or in the liquid form.The color concentrate includes a colorant. The colorant may be apigment, a dye, a combination of pigment and dye, or other coloredmaterial. The precolored yellow material is mixed/blended with a blueconcentrate to form fabricated plastic parts which are green.

The first material 100 is fed into the machine using the first hopper 52and the second material 102 is fed into the machine using a volumetricfeeder or a gravimetric feeder 54. In another embodiment, the firstmaterial and second material may be premixed prior to loading in thefirst hopper 52. For example, the first material may be yellow plasticpellets which are premixed with blue plastic pellets to create a mixtureof yellow pellets and blue pellets.

The first and second materials 100, 102 are advanced through the screw58 and barrel 56 of the machine. As the materials are advanced, thematerials melt and are mixed together to form a resultant third material104 with a third color. The resultant third material 104 is eitherextruded to form a part or is injected into a mold to form a part.

The color of the part is then measured subjectively, or, more preferablyobjectively, with any suitable device. Illustrative examples of usefulmeasuring devices include spectrophotometers, 3 or 4 filter tristimuluscolorimeters, spectrocolorimeters, electronic video systems, or stillframe camera-based systems.

The color may be measured by any system desired, preferably using L*a*b*values which are known by persons skilled in the art. It is known thatcolor applied to a surface can be defined in terms of a color in athree-dimensional color space or color map. The ideal color space is acylinder. The color sphere is defined by the lightness (“L”) value, thered/green (“a”) value, and the blue/yellow (“b”) value. See, forexample, Color Measurements Instrument Manual, X-Rite® No. 948-968(1990) and Commission International de l'Eclairage as the CIE 1976L*a*b* Color Space. Thus, the three values when combined define a colorof an object as seen by the human eye. Any given color can then bemeasured in terms of the “L*a*b*” value and expressed in relation to theL*a*b* value of a prior measurement. In addition, color may be expressedby C* (chroma) or by h (hue angle). The value C* equals$\sqrt{a^{*2} + b^{*2}}.$

The value h equals the arctangent of b*/a*. Furthermore, color changecan be expressed by ΔE which equals$\sqrt{\left( {L_{1}^{*} - L_{2}^{*}} \right)^{2} + \left( {a_{1}^{*} - a_{2}^{*}} \right)^{2} + \left( {b_{1}^{*} - b_{2}^{*}} \right)^{2\quad}}.$

The color of the fabricated plastic parts will change as the screw 58and barrel 56 wear. Specifically, color changes because less mixingoccurs as the distance 70 between the screw 58 and the barrel 56increases causing the colorants of the second material to become lesshomogeneously distributed in the third resultant material.

The color value would then be compared with previous color values todetermine the amount of wear. Specifically, the method would beperformed on the machine 50 when the machine is new (or first used orrepaired) in order to obtain a first or initial color value 120. In thisexample, the color value is C* (chroma). Referring to FIG. 2, the firstvalue 120 would be placed on a graph 122 with operating time 124 alongthe X axis and color 126 along the Y axis. After the specified intervalof operating time 128, the method would be performed again to obtain asecond color value 130 and this second value 130 would be placed on thegraph 122. Similarly, the method would be performed after eachadditional interval of operating time 132, 134 to obtain the third value136, fourth value 138, etc. The values would then be connected with aline to form a curve 142. In this example, the first material is aprecolored compound and the second material is a color concentrate. Thefirst color is yellow, the second color is blue and the resultant thirdcolor is green. The curve moves upward because the yellow becomes moredominant in the third color and makes the resultant third (green) colorbrighter and more yellow. The user would analyze the curve 142 todetermine the time when the color, and thus wear, would reach apredetermined unacceptable value 144. The machine would then bedisassembled and rebuilt at the specified time.

In other examples, and depending upon the colors used, the color valuea*, b*, L*, h, or ΔE may be used as the color value to be placed uponthe graph. For example, referring to FIG. 3, the graph 150 shows anexample where the color value is b* and the curve 152 moves downward. Inthis example, the first material is a precolored compound and the secondmaterial is a color concentrate. The first color is white, the secondcolor is blue and the resultant third color is light blue. The curve 152moves upward because the white becomes more dominant in the resultantthird color and makes the resultant third color duller and more white.

In another embodiment, the color values would be compared to the colorvalue of a color chip. In an additional embodiment, the color valueswould be compared to the color value of a control part with a specifiedcontrol color.

In another embodiment, a mathematical equation would be created whichrepresents the relationship between the color values and the timeperiod. The equation would be used to determine the time when the colorwill reach a predetermined value. The machine would then be disassembledand rebuilt at the specified time. The equation could be created using acomputer and appropriate software.

In another embodiment, the color is measured at several locations on thepart and the values are compared with a standard for differences. If thedifferences in color values at the locations are unacceptable, thenthese differences would indicate wear in the machine. As the machinewears, the mixing decreases and the difference between color values atvarious locations increases. For example, referring to FIG. 4, the coloris measured at five locations, 160, 162, 164, 166, 168 on a specifiedarea on the part 170. The color values are compared with a standard andif unacceptable differences in color values exist, then the machinewould be disassembled and rebuilt.

In another embodiment, a graph or equation is created for a particularmodel number of a machine by a specific manufacturer. For example, asshown in FIG. 5, a graph 200 is created for a machine by manufacturer Xwith model number Y. The graph may have several lines for different wearrates depending on the abrasiveness of the first and second materials.For example, the graph 200 may have a first line 202 for low abrasivematerials, a second line 204 for medium abrasive materials, and a thirdline 206 for high abrasive materials. The manufacturer or a servicecompany would create the graph for a specific model number using themethod noted above. After a specific period of operating time, themethod would be performed to obtain the color value 210 of the thirdcolor. After obtaining the color value 210, the appropriate line isselected, e.g. medium abrasive materials 204, and the color value 210and time 212 are located on the line 204. By knowing the location on theline, the time 214 when the machine would reach the predetermined value216 of unacceptable color can be determined. This method could also beperformed using equations as noted above.

In a further embodiment, the first material with a first color and thesecond material with a second color are processed in the machine toobtain a resultant third material with a third color. The machine isthen purged or cleaned. A fourth material is then processed in themachine. The fourth material contains the same colorant at the samepercentage as the resultant third material, except the fourth materialis completely homogeneous prior to processing in the machine. Thus, thefourth material with the fourth color will become the homogeneous colorstandard. More specifically, the fourth color represents a homogeneouscolor standard for the particular thermal history and operatingconditions of the machine. If the machine has been in use for a periodof time, then this color standard (resultant fourth color) wouldrepresent the machine when it was new and was providing a completelyhomogeneous output. The resultant third color is then compared with thecolor standard (resultant fourth color) to determine the differencebetween the color values and thus, the wear.

In another embodiment, the method may be performed on a new machine oron a rebuilt machine in order to certify that the machine is performingat an acceptable level. Specifically, the first material with a firstcolor and the second material with a second color are processed in themachine to obtain a resultant third material with a third color. Thethird color is measured and compared to a color chip, a control partwith a control color, and/or with a fourth color as noted herein. Inaddition, the fourth material with the fourth color may be processed inthe machine to obtain a resultant fourth color for the particularthermal history and operating conditions of the machine. The resultantfourth color is then compared with other color values as noted herein.If the machine is new, then the resultant third color and/or resultantfourth color may be compared with color values of other new machines. Ifthe machine has been rebuilt, then the resultant third color and/orresultant fourth color may be compared with color values obtained priorto rebuilding the machine.

Preferably, the first material should have an opaque first color.However, in another embodiment, the first material may be a naturalresin with a natural first color and the second material may be a colorconcentrate with a second color. The natural resin with the naturalfirst color is relatively transparent and provides a poorer base for thesecond color as opposed to an opaque base. However, the color value ofthe resulting third color can be measured and the color can be comparedwith previous color values to determine the amount of wear. In thisembodiment, the user can continue to run saleable parts with the usualcolor, as opposed to parts with a special color for the wear test whichwould not be saleable due to the special color. Indeed, by adopting atest protocol based upon the color and part most normally made on themachine being tested, the timing of the testing can be more easilyscheduled, causing little, if any, disruption to the desired productionschedules. In yet another embodiment, the first material may be anatural resin with a natural first color and the second material may bea colorant with a second color. The colorant may be a pigment, a dye, acombination of pigment and dye, or other colored material. The pigmentmay be dry or a paste.

Any combination of colored materials, whose calorimetric values (all orany) would be sensitive to change, may be used. For example, other colorcombinations may be used as shown in the table below.

Third Color of First Color of Second Color of of Resultant FirstMaterial Second Material Third Material Red Yellow Orange Red Green GreyGreen Blue Blue Green Blue Red Violet White Black Grey White Blue LightBlue

Certain combinations have greater sensitivity to non-homogeneity.

In order to obtain accurate results for the color values, as is desiredin the most preferred embodiment of this invention, so as to achieve themaximum benefits, all variables should be eliminated or controlled. Forexample, the first and second materials should be the same at eachinterval measurement. Specifically, the first material should be thesame product from the same manufacturer for each interval measurement.Similarly, the second material should be the same product from the samemanufacturer for each interval measurement. The first material and thesecond material should be of a uniform and consistent pellet size (orparticle size) and configuration. In addition, the color (hue), thestrength (chroma), and the shade of the first and second materialsrequire tight color tolerance control. If a different product and/ormanufacturer is used, variables are introduced into the method, such as,the size and composition of the pellets. Furthermore, the same moldshould be used for each interval measurement. If a different mold isused, variables are introduced into the method, such as, differences insurface texture. In addition, the operation of the machine should becontrolled, for each interval measurement, such as, the back pressure,the screw speed, holding time, dwell time, cylinder temperature, size ofnozzle and nozzle shape.

The method may be performed by a service company as a service for theuser of the machine. The service company would perform the intervalmeasurements and provide the data and recommendations to the user.Inasmuch as precolor services are available and being used to providecolor concentrates for normal production, such services have theexpertise to provide appropriate test protocols and color samples.

In some situations, the method may be completely performed by the userif the user has the skills and equipment. In other situations, somesteps in the method may be performed by the user, while other steps maybe performed by the service company. For example, the service companycan provide the user with the first and second materials separately orin a premixed form. The user then performs the method at the specifiedtime interval and sends the fabricated part to the service company foranalysis, including measurement of the color intensity value. Theservice company then graphs, or otherwise evaluates, the values andprovides the data and recommendations to the user.

In another embodiment, the user supplies the first and second materialsand performs the method at the specified time interval. The user thensends the part to the service company for analysis, includingmeasurement of the color intensity value. The service company thengraphs, or otherwise evaluates, the values and provide the data andrecommendations to the user.

In yet another embodiment, the method may be performed on a new machineby the manufacturer or on a rebuilt machine by the rebuilder in order tocertify that the machine is performing at an acceptable level.

While certain features of the invention have been illustrated anddescribed herein, modifications, substitutions, changes, and equivalentsmay occur to those skilled in the art. It is, therefore, to beunderstood that the appended claims are intended to cover all suchmodifications and changes as fall within the true spirit of theinvention.

What is claimed is:
 1. A method for determining wear in a machinecomprising: a. providing a first material of a first color; b. providinga second material of a second color; c. melting the first and secondmaterials in the machine to obtain a third material of a third color; d.obtaining the color of the third color to obtain a first color value; e.obtaining a comparison of the first color value with a second colorvalue, a difference between the first color value and the second colorvalue indicates wear in the machine.
 2. The method as in claim 1 whereinthe second color value is obtained from a color chip.
 3. The method asin claim 1 wherein the second color value is obtained from a controlpart.
 4. The method as in claim 1 wherein the second color value isobtained from a predetermined line on a graph.
 5. The method as in claim1 wherein the second color value is obtained from a predeterminedmathematical equation.
 6. The method as in claim 1 wherein the machinehas a screw and a barrel which are separated by a distance, thedifference between the first color value and the second color valueindicates a change in the distance and wear in the machine.
 7. Themethod as in claim 1 further comprising: repeating steps a-c after aspecified period of time and obtaining the color of the third color toobtain a second color value.
 8. The method as in claim 7 furthercomprising the step of creating a graph with color value on one axis andtime on the other axis, placing the first color value and second colorvalue on the graph and forming a line with the values.
 9. The method asin claim 8 further comprising the step of extrapolating the line beyondthe values to determine the time when the color value will reach apredetermined value.
 10. The method as in claim 7 further comprising thestep of repeating steps a-c after a second specified period of time andobtaining the color of the third color to obtain a third color value.11. The method as in claim 10 further comprising the step of repeatingsteps a-c after a third specified period of time and measuring the colorof the fourth color to obtain a fourth color value.
 12. The method as inclaim 10 further comprising the step of creating a graph with colorvalue on one axis and time on the other axis, placing the color valueson the graph and forming a line with the values.
 13. The method as inclaim 12 further comprising the step of extrapolating the line beyondthe values to determine the time when the color value will reach apredetermined value.
 14. The method as in claim 7 further comprisingcreating a mathematical equation which represents the relationshipbetween the values and the time period.
 15. The method as in claim 14wherein the equation is used to determine the time when the color valuewill reach a predetermined value.
 16. The method as in claim 1 whereinthe first material is in pellet form and the second material is inpellet form.
 17. The method as in claim 16 wherein the first materialand second material are premixed.
 18. The method as in claim 1 whereinthe first material is in pellet form and the second material is inliquid form.
 19. The method as in claim 1 wherein the first material isa precolored compound and the second material is a color concentrate.20. The method as in claim 19 wherein the first color is yellow, thesecond color is blue and the third color is green.
 21. The method as inclaim 19 wherein the first color is white, the second color is blue andthe third color is light blue.
 22. The method as in claim 1 wherein thefirst material is a natural resin and the second material is a colorconcentrate.
 23. The method as in claim 1 wherein the first material isa natural resin and the second material is a colorant.
 24. The method asin claim 23 wherein the colorant is selected from the group consistingof: a pigment, a dye, and a combination of a pigment and a dye.
 25. Themethod as in claim 1 wherein the machine is an extruder.
 26. The methodas in claim 1 wherein the machine is an injection molding machine. 27.The method as in claim 1 wherein the third material is molded into apart.
 28. The method as in claim 27 wherein the step of obtaining thecolor is performed on the part.
 29. The method as in claim 1 whereinstep d is performed using a measuring device.
 30. The method as in claim29 wherein the measuring device is a spectrophotometer.
 31. Theinvention as in claim 4 wherein the predetermined line is obtained bymeasuring the color of the third material at various intervals of time.32. The invention as in claim 4 wherein the predetermined linecorresponds to the abrasiveness of the first material or the secondmaterial.
 33. The invention as in claim 32 wherein a secondpredetermined line corresponds to the abrasiveness of a different firstmaterial or second material.
 34. The invention as in claim 5 wherein thepredetermined equation is obtained by measuring the color of the thirdmaterial at various intervals of time.
 35. The invention as in claim 5wherein the predetermined equation corresponds to the abrasiveness ofthe first material or the second material.
 36. The invention as in claim35 wherein a second predetermined equation corresponds to theabrasiveness of a different first material or second material.
 37. Amethod for determining wear in a machine comprising: a. providing afirst material of a first color; b. providing a second material of asecond color; c. melting the first and second materials in the machineto obtain a third material of a third color; d. forming the thirdmaterial into a part; e. obtaining the color of the third color at afirst location on the part to obtain a first color value and obtainingthe color of the third color at a second location on the part to obtaina second color value; f. obtaining a comparison of the first color valuewith the second color value.
 38. The method as in claim 37 wherein themachine has a screw and a barrel which are separated by a distance, thedifference between the first color value and the second color valueindicates a change in the distance and wear in the machine.
 39. Themethod as in claim 37 wherein the first material is in pellet form andthe second material is in pellet form.
 40. The method as in claim 39wherein the first material and second material are premixed.
 41. Themethod as in claim 37 wherein the first material is in pellet form andthe second material is in liquid form.
 42. The method as in claim 37wherein the first color is yellow, the second color is blue and thethird color is green.
 43. The method as in claim 37 wherein the machineis an extrusion molding machine.
 44. The method as in claim 37 whereinthe machine is an injection molding machine.
 45. The method as in claim37 wherein the third material is molded into a part.
 46. The method asin claim 37 wherein step e is performed using a measuring device. 47.The method as in claim 46 wherein the measuring device is aspectrophotometer.
 48. A method for determining wear in a machinecomprising: a. providing a first material of a first color; b. providinga second material of a second color, which will be melted with the firstmaterial in the machine to obtain a third material of a third color; c.obtaining the color of the third color to obtain a first color value; d.obtaining a comparison of the first color value with a second colorvalue, a difference between the first color value and the second colorvalue indicates wear in the machine.
 49. A method for determining wearin a machine using a first material of a first color and a secondmaterial of a second color which are melted in the machine to obtain athird material of a third color, the method comprising: a. at a firstperiod of time, obtaining a first sample of the third material andmeasuring the color of the third color to obtain a first color value; b.obtaining a comparison of the first color value with a second colorvalue, a difference between the first color value and the second colorvalue indicates wear in the machine.